Method of removing iron oxide deposits from the surface of titanium components

ABSTRACT

Disclosed is a method and solvent composition capable of removing iron oxide deposits from the surface of titanium components without substantially damaging the underlying titanium component. Iron oxide deposits may be removed from the surface of a titanium component by contacting the titanium component with the solvent composition of the invention. The solvent composition may then be removed from contact with the titanium component to obtain a recyclable solvent composition which is recycled into repeated contact with the titanium component. The solvent composition comprises an aqueous mixture of an organic acid and a hydrohalide acid.

This application claims the benefit of U.S. Provisional PatentApplication No. 60/327,464, filed Oct. 5, 2001, which is incorporatedherein by reference. This application is a Divisional of Ser. No.10/255,339, filed Sep. 26, 2002, now U.S. Pat. No. 6,852,879.

FIELD OF THE INVENTION

The present invention relates to a method and composition for removingiron oxide deposits from the surface of titanium components, and moreparticularly the use of such a method and composition to reduce orcontrol buildup of iron oxide on titanium surfaces of process equipmentin chemical manufacturing processes in which liquid process streams thatmay contain impurities in the form of dissolved iron or iron compoundsare present and come in contact with such process equipment.

BACKGROUND OF THE INVENTION

Titanium components (defined as equipment and components of equipmentmade from, coated with or clad with titanium metal or otherwise having asurface of titanium metal) are useful in a variety of systems, processesand environments in which corrosion resistance is important. Forexample, titanium metal is used as a material of construction orfabrication for reaction vessels, liners and other reactor internals inchemical and other industrial processes that use or involve exposures tocorrosive materials (including solvents, reactants, and by-products) orenvironments, such as an oxidizing environment. Titanium components alsoinclude heat exchangers due to titanium's corrosion resistance andresulting extended life of the equipment. Titanium components in theform of packing materials are often used in distillation columns andother separation devices used for gas-liquid separations involvingcorrosive materials. Although titanium components themselves arecorrosion resistant, iron oxide deposits may form on surfaces of suchtitanium components when they are employed in systems or processes inwhich they come in contact with a source of iron, such as soluble ironor iron compounds which may be present as impurities in process streams.For example, titanium components are often integrated into systems whichalso employ relatively inexpensive stainless steel materials in areaswhere corrosion resistance is less important. These stainless steelmaterials may introduce dissolved iron into the system, which canaccumulate on the surface of titanium components as iron oxide. In othersystems, possible sources of iron include catalysts, liquid processstreams and impurities.

The accumulation of iron oxide on a titanium component surface is oftengradual but amounts of iron oxide accumulated over time can affect theusefulness of the titanium component. In the case of a titaniumcomponent in the form of a heat exchanger, for example, the accumulationof iron oxides on the surface of the titanium component may interferewith heat transfer. When titanium components are used in a distillationcolumn as packing materials, a thin layer of iron oxides may form on thesurface of the titanium packing when repeatedly exposed to dissolvediron, and it has been reported that “Accumulations of iron oxide . . .on titanium structured packing can promote or accelerate combustion oftitanium. It may be appropriate to periodically remove accumulations ofsuch materials through chemical or other means. However, if removal isaccompanied by loss of titanium, it can create thinner metal, which maybe sensitive to ignition” (Centerline, Vol. 5, No. 2, Summer 2001, pp.6–8, 15–18, Mary Kay O'Connor Process Safety Center). This publicationalso reports that the presence of iron oxides “accelerated the oxidationof the titanium [packing] via a mechanism known as the Thermite Reactionin which the oxygen for combustion is taken from a less reactive metaloxide.” Examples of thermite type reactions involving titanium metal andiron oxides may be represented by the following: 2FeO+Ti→TiO₂+2Fe; or2Fe₂O₃+3Ti→3TiO₂₊₄Fe.

Methods and compositions for removing oxide deposits are known from U.S.Pat. No. 3,957,529, which discloses a cleaning solvent comprisingsulfuric acid and citric acid and its use to clean metal surfaces; U.S.Pat. No. 4,174,290, which discloses a method for removing metal oxideswith a composition comprising an amine, a strong mineral acid and citricacid; and U.S. Pat. No. 4,250,048, which discloses a method for removingmetal oxides with a composition comprising an ammonia derivative such asan amine, a strong mineral acid and an organic chelating agent for themetal oxides in an aqueous solution with a pH of about 0.5–3.0. U.S.Pat. No. 4,623,399 discloses a method of removing iron oxide scale frommetal surfaces with a composition comprising a hydroxyethylethylenediamine triacetic acid and an organic acid such as formic acid.It also has been reported that ferrometal corrosion products are removedby use of citric acid or citric acid-tannin complexing agents witherythorbic acid reducing agents.

Although traditional methods and compositions are often useful forremoval of iron oxides from various surfaces, their utility inparticular applications can be limited in various respects, such asinadequate selectivity to iron oxides over other desirable metals thatmay be present and lack of compatibility or difficult integration withother aspects of the application. In the case of iron oxide deposits onsurfaces of titanium components, selective removal, such that iron oxideis removed without substantial damage to or dissolving of the titaniumcomponent, can be especially important due to the relatively high costof the titanium components.

It would, therefore, be desirable to provide a method and compositionfor removing iron oxide deposits from a titanium component withoutdamaging the titanium component. In the case of titanium components usedin chemical and other industrial processes, it would be particularlydesirable to provide for removal or control of surface deposits of ironoxide on titanium components integrated with other aspects of theprocesses.

SUMMARY OF THE INVENTION

We have discovered a method and solvent composition capable ofselectively removing iron oxide deposits from the surface of titaniumcomponents.

In accordance with the invention, the solvent composition comprises anaqueous solution of an organic acid and a hydrohalide acid.

In accordance with the invention, iron oxide deposits are selectivelyremoved from a titanium component surface by contacting the titaniumcomponent with the solvent composition of the invention. Contacting withthe solvent composition can be carried out in a single pass or byrecycling the composition, including dissolved species, into one or morerepeated contacts with the titanium component.

In accordance with another embodiment of the invention, iron oxidedeposits which may be present are selectively removed from the surfaceof titanium component internals of a gas-liquid separation apparatus,such as a distillation column, by contacting the titanium component orcomponents with a solvent composition comprising an aqueous solution ofan organic acid and a hydrohalide acid in the substantial absence ofmolecular oxygen. Preferably in such an embodiment, aqueous organic acidis heated and introduced into the apparatus and circulated therein, andaqueous hydrohalide acid is added to the aqueous acid and the result ispassed through the separation device. In a preferred embodiment, theorganic acid is heated in contact with a heat exchange surface beforebeing introduced into the separation device and before addition ofsubstantial hydrohalide acid thereto, the heated acid is introduced intoand circulated through the separation device in contact with itstitanium component internals, the heat exchange surface is allowed tocool to a temperature low enough to avoid damage thereto on exposure tohydrohalide acid, the hydrohalide acid is added to the organic acid toform the solvent composition of the invention, and the solventcomposition is passed through the separation device in contact with itsinternal titanium components.

The present invention is particularly useful for selectively removingsolid iron oxides from the titanium component internals of adistillation column where the internals of the distillation column havebeen exposed to a source of iron (typically dissolved iron), andespecially when also exposed to an oxidizing agent or environment, suchas exposure to molecular oxygen.

BRIEF DESCRIPTION OF THE DRAWING

The single FIGURE of the drawing depicts an embodiment of the inventionwherein iron oxide deposits may be removed from the titanium componentinternals of a distillation column.

DETAILED DESCRIPTION OF THE INVENTION

The invention is based, in part, on the discovery that iron oxidepresent in solid form on surfaces of titanium packings from adistillation column used for separation of water and acid in overheadgases generated in a liquid phase oxidation process for making aromaticcarboxylic acids, and believed to have accumulated on the packings as aresult of precipitation or deposition from vaporized process liquids ordistillation reflux containing dissolved iron compounds, have aconsiderably higher content of iron (III) oxide, and particularly alphaFe₂O₃, than iron (II) oxide. While iron-containing impurities dissolvedin various liquid streams used in the process are believed to have ahigher content of iron (II), the oxidizing agents, high temperatures andacidic and corrosive environment of the process promote oxidation of themore soluble iron (II) species dissolved in the liquid streams to lesssoluble iron (III) oxide, which tends to deposit on metal surfaces ofprocess equipment. Titanium components find substantial use in suchprocesses due to the severity of process conditions and materials, andaccordingly, removal of iron oxide deposits which may form, andparticularly iron (III) oxides, without damage to or dissolving titaniumof the components, is important for process operation and equipmentmaintenance.

It is believed that the limited effectiveness of known methods andcompositions to selectively remove iron oxides from titanium componentsurfaces may be attributed to iron (III) oxide. While iron (III) oxidesas well as iron (II) oxide are often present in iron oxides formed onthese titanium component surfaces, the former may predominate or bepresent in substantial amounts. Solvents that are effective for removalof deposited iron (II) oxide are less effective for dissolving depositediron (III) oxide, and thus, removal of iron (III) oxides typicallyrequires stronger solvents compared to iron (II) oxides. Althoughcapable of completely or substantially removing iron oxides from atitanium component surface, stronger solvents also dissolve unacceptablyhigh levels of titanium metal from the component and can cause damage toit. By use of the present invention's method and solvent composition,iron oxide deposits may be selectively removed from a titanium componentsurface even when substantial iron (III) oxide is present.

In accordance with one aspect, the invention provides a solventcomposition useful for selectively removing iron oxides from titaniumcomponent surfaces comprising a hydrohalide acid, water, and an organicacid. Preferably, the solvent composition consists essentially ofhydrobromic acid, water, and an organic acid. The solvent composition iscapable of selectively removing substantial amounts of iron oxides,including all visible amounts of iron oxide deposit, from a titaniumcomponent surface. Removal of iron oxides is accomplished without orwith only insubstantial removal of titanium from the titanium component.

In one embodiment, the solvent composition preferably has a titaniumuptake capacity of no more than 2000 ppmw, more preferably no more than500 ppmw, still more preferably no more than 100 ppmw, and even morepreferably no more than 50 ppmw, and most preferably no more than 15ppmw. Titanium uptake capacity refers to the amount of titanium metalwhich a given solvent composition is capable of dissolving on contactwith a titanium metal sample at conditions under which the solvent is tobe used. For preferred uses of the method and solvent composition of theinvention, titanium uptake capacities are conveniently determined basedon contact times of at least 48 hours at 80° C.

In another embodiment, the solvent composition can be used to remove allor substantially all visible iron oxide deposits from a titaniumcomponent surface while removing less than 1 wt. % of the total titaniummetal in the titanium component. Preferably, visible iron oxide depositsfrom a titanium component surface are at least substantially removedwhile removing less than 0.5 wt. %, and still more preferably, less than0.25 wt. %, of the total titanium metal in the titanium component.

The amounts of hydrohalide acid, water and organic acid present in thesolvent composition are chosen such that the composition is capable ofselectively removing iron oxide deposits from a titanium componentsurface. If too much hydrohalide acid is present, the solventcomposition typically can dissolve an undesirable amount of the titaniumin the titanium component. Accordingly, the amount of hydrohalide acidis preferably chosen such that the resulting solvent composition has atitanium uptake capacity of no more than 2000 ppmw, preferably no morethan 500 ppmw, more preferably no more than 100 ppmw, and even morepreferably no more than 50 ppmw, and even more preferably no more than15 ppmw. Suitably, hydrohalide acid is present in the solventcomposition in an amount between 0.5 to 20 wt. %, preferably from about1 to about 10 wt. %, and more preferably from 4 to 8 wt. % of the totalweight of the solvent composition. Preferably the hydrohalide acid ishydrobromic acid or hydrochloric acid, with hydrobromic acid being mostpreferred.

The amount of water present in the solvent composition is chosen suchthat the resulting solvent composition is effective for the selectiveremoval of iron oxides from a titanium component surface. At relativelyhigh levels of water, the solvent composition typically loses itseffectiveness in removing iron oxide deposits. Water is suitably presentin the solvent composition in an amount between 0.5 to 25 wt. % morepreferably from 3 to 15 wt. %, and even more preferably from 4 to 8 wt.% of the total weight of the solvent composition.

After the appropriate amounts of hydrohalide acid and water have beenchosen, the remainder of the solvent composition comprises primarily anorganic acid. Preferably, the remainder of the solvent compositionconsists essentially of, and more preferably, consists of an organicacid.

In one preferred embodiment, the solvent composition compriseshydrohalide acid in an amount of 4–8 wt. %, water in an amount of 4–8wt. %, with the remainder, ranging from about 84 to about 92 wt. %,being an organic acid.

The organic acid present in the solvent composition is an alkylmonocarboxylic acid having from 2 to 6 carbon atoms, benzoic acid, or amixture thereof. Examples of suitable aliphatic carboxylic acids includeacetic acid, propionic acid, butanoic acid and hexanoic acid. Aceticacid is preferably used as the organic acid because it is relativelyinexpensive and readily recycled, i.e. easy to separate from othercomponents.

For use in chemical or industrial processes in which process equipmentcomprising one or more titanium components contacts process streams ormaterials in which iron-containing impurities may be dissolved orpresent and in which reactants, solvents, products or other processmaterials or intermediates include an organic acid suitable for use inthe solvent compositions used according to the invention, it isadvantageous for efficient materials usage and recycle operations if theacid included in the solvent composition is the same as the processacid. For example, in the liquid phase oxidation of feed materialscomprising an aromatic hydrocarbon to an aromatic acid using acetic acidas reaction solvent, use of acetic acid as the organic acid component ofthe invented solvent composition can eliminate or reduce redundancies inhandling, separation, purification, recycle and other equipment andsystems used for the acetic acid.

The amount of solvent composition used to selectively remove iron oxidedeposits from a given titanium component surface may be determined byfactors including iron uptake capacity of the solvent composition, theamount of iron present in the iron oxide deposits and extent to whichiron is to be removed. Iron uptake capacity refers to the maximum amountof iron, calculated on the basis of elemental iron, which a givensolvent composition is capable of dissolving when contacted with ironunder conditions of use. As with titanium uptake capacity, for preferreduses of the solvent according to this invention, iron uptake determinedbased on contact times of at least 48 hours and temperatures of about80° C. are convenient. Thus, for some applications it may be practicalto determine the amount of solution of known iron uptake capacity to beused by approximating the amount of iron oxide deposits to be removedand the amount of iron contained in the iron oxides, and calculating theamount of solvent composition to be used.

The solvent composition of the invention is useful for selective removalof both iron (II) oxide (FeO), iron (III) oxides, including Fe₂O₃ inboth alpha and gamma forms and FeO(OH), and the mixed oxide Fe₃O₄. It isparticularly useful for selectively removing iron oxides comprising iron(III) oxides, and especially alpha Fe₂O₃, without dissolving substantialtitanium from the titanium component. Iron (III) oxides typically aremore difficult to selectively remove compared to iron (II) oxides. Thesolvent composition of the invention is capable of selectively removingiron oxide deposits from titanium components wherein the iron oxidedeposits comprise primarily iron (III) oxide. Iron oxide depositscomprising at least 60 wt. % iron (III) oxide, and even as high as atleast 90 wt. % are effectively removed or reduced according to theinvention. Accordingly, the invention is particularly useful forreducing or controlling accumulation of iron oxide deposits on titaniumcomponents used in processes in which iron-containing impuritiesdissolved in process liquids or streams are exposed to the components inthe presence of oxidizing reagents or atmospheres capable of oxidizingsoluble iron (II) species to iron (III) oxides.

The solvent composition of the invention may be used to selectivelyremove iron oxide deposits formed on titanium component surfaces bycontacting such titanium components with the solvent composition.Preferably for some applications, after the solvent composition hascontacted the titanium component it is recycled and is again used tocontact the titanium component in order to selectively remove additionaliron oxide deposits. The solvent composition may be recycled in abatch-wise or continuous mode until the desired amount of iron oxidedeposit has been removed. Within limits of a given solution's ironuptake capacity under conditions of use, each successive recyclingremoves an additional amount of the iron oxide deposits. Preferably, thesolvent composition is recycled until all visible iron oxide depositsare removed from the titanium component while removing less than 1 wt.%, preferably less than 0.5 wt %, more preferably less than 0.25 wt. %of the total titanium in the titanium component.

When contacting the titanium component surface, the temperature of thesolvent composition, whether fresh or recycled solvent that may alsocontain amounts of dissolved iron, should be chosen so that the solventcomposition continuously remains homogenous and in the liquid phase butdoes not damage the titanium component or other equipment that may alsobe exposed to the solvent. Preferably, the temperature is sufficientlyhigh to maximize the solubility of iron oxides deposits in the solventcomposition or recycle thereof while sufficiently low so as to minimizedissolving the titanium of the titanium component. Preferably, thetemperature should be sufficiently low so that less that 1 wt %,preferably less than 0.5 wt. %, even more preferably less than 0.25 wt.% of the total titanium in the titanium component is dissolved by thesolvent composition upon removal of all visible iron oxide deposits fromthe titanium component. Temperatures below about 135° C. are preferredfor solvent compositions containing hydrobromic acid because highertemperatures can lead to dissolution of undesirable levels of titaniummetal. When contacting the titanium component, temperatures for thesolvent composition or recycle thereof suitably range from about 10 toabout 135° C., and preferably include between 10° C. to 125° C., morepreferably between 55° C. to 100° C., and even more preferably between70° C. to 90° C.

The pressure at which the titanium component is contacted with thesolvent composition is any pressure which is suitable for a chosentemperature. Using above atmospheric pressures allows for highertemperature ranges for the solvent composition or recycle thereof. Usingbelow atmospheric pressures requires the use of lower temperatures.Preferably, the pressure at which the titanium component is contactedwith the solvent composition is about atmospheric.

The solvent composition or recycle thereof is preferably contacted withthe titanium component surface in the substantial absence of molecularoxygen in order to avoid formation of potentially flammable mixtures. Anatmosphere substantially free of molecular oxygen is convenientlyprovided by maintaining an atmosphere or flow of inert gas such asnitrogen around or within the titanium component to be treated.

Equipment, such as tanks, pumps, heat exchangers and piping, that may beused for mixing and heating the solvent composition and conveying it toor from a titanium component to be treated with the composition shouldbe constructed from materials which themselves do not contain solubleforms of iron that might be dissolved by the composition, therebyreducing its iron uptake capacity before use. Stainless and other steelsand other iron-containing metals are particularly disadvantageous inthis regard. Titanium equipment is well suited for use as is equipmentfabricated from polyethylene, polypropylene, polyvinylidine fluoride andother plastic resins that are resistant to attack by the solvent.

According to a more specific embodiment of the invention, an aqueoussolvent composition comprising hydrogen halide acid and an aliphaticcarboxylic acid of 2 to about 6 carbon atoms, benzoic acid or acombination thereof is used for removal or control of solid iron oxidedeposits on titanium components, and particularly titanium components ofdistillation or other gas-liquid separation equipment, used in processesfor the manufacture of aromatic carboxylic acids. In such processes,feed material comprising an alkyl aromatic compound or other oxidizablearomatic hydrocarbon, a catalyst composition, solvent comprising amonocarboxylic acid (e.g. acetic acid) and water are charged to areaction vessel. Air is introduced into the reactor in order to providea source of oxygen (O₂) necessary to complete a catalytic liquid phaseoxidation which converts the alkyl aromatic to an aromatic acid. Whileair is a preferred source of molecular oxygen, pure oxygen,oxygen-enriched air and other sources also are suitable. The oxidationof aromatic hydrocarbon feed materials to product comprising Aromaticacid is conducted under oxidation reaction conditions. Temperatures inthe range of about 120 to about 250° C. are suitable, with about 150 toabout 230° C. preferred. Pressure in the reaction vessel is at leasthigh enough to maintain a substantial liquid phase comprising feed andsolvent in the vessel. This will vary with vapor pressures of the feedand solvents used in a given operation; by way of example, in themanufacture of terephthalic acid by oxidation of feed comprisingpara-xylene and solvent comprising acetic acid, suitable gauge pressuresin the reaction vessel are about 0 to about 35 kg/cm² and preferablyabout 10 to about 20 kg/cm².

The oxidation produces water, at least a part of which typically isremoved from the system. The oxidation is exothermic and the liquidreaction mixture normally is maintained in a boiling state fordissipation of heat of the reaction by vaporizing volatile components ofthe liquid mixture. This produces an overhead gas in the reactionvessel, which preferably is removed at the top of the vessel. Thisoverhead gas comprises vaporized monocarboxylic acid, water vapor,oxygen (O₂), and gaseous by-products of the oxidation. When air is usedas a source of molecular oxygen, nitrogen also is present in theoverhead gas phase. In order to remove water while recycling othercomponents (e.g. monocarboxylic acid) back to the reaction, the overheadgas is removed from the reactor to a separation device such as adistillation column or other reflux condenser. The separation deviceincludes internal titanium components, such as packing or trays, due tocorrosivity of the overhead gas. An example of titanium componentpacking materials are titanium packing materials, Titanium grade 1, 0.10mm thickness, GEMPAK 2A commercially obtainable from Kock-Glitsch Inc.of Wichita, Kans.

In distillation, the internal titanium component packing materialsfacilitate the separation of monocarboxylic acid from water so that themajority of the acid condenses from the overhead gas and segregates tothe bottom of the column and can be returned to the reaction vesselwhile a second gas phase comprising water vapor and uncondensablespecies is removed from the top of the column. Packings typically are inthe form of thin sheets of titanium metal, which are typicallycorrugated, and disposed side-by-side in bundles within the column toprovide a high surface area for heat exchange between an upwardlyflowing vapor phase and downwardly flowing liquid phase comprisingreflux and condensate. The distillation uses a water reflux, which cancome from any source but preferably uses water from the system. Forexample the water removed from the top of the distillation column can becondensed and a portion of this condensate may be sent to thedistillation column as reflux. The water reflux may also come from waterwhich is used to wash or purify the aromatic acid product such asdescribed in U.S. Pat. No. 5,723,656. Water reflux may be a source ofdissolved iron resulting from contact with steel components of otherequipment used in the liquid phase oxidation process or downstreamprocess steps such as product recovery and purification. Examples ofthese steel components include piping, crystallizers, storage tanks,driers, centrifuges, filters, condensers, pumps, scrubbers, and thelike. As described above, it is believed that the iron of the waterreflux (thought to contain primarily Fe⁺²) reacts with the oxygen of theoverhead gas to form predominately iron (III) oxide on the surface ofthe packing materials. Internal packing materials exposed to thedissolved iron-containing water reflux and oxygen-containing overheadgas were analyzed by x-ray diffraction (XRD) and energy dispersivespectrometry (EDS), indicating that up to about 90 wt. % of the solidiron oxide deposits comprised iron (III) oxide with the remainder iron(II) oxide.

Iron oxide deposits are removed from internal titanium components ofseparation equipment, such as distillation column packings, used inprocesses for manufacture of aromatic acids by steps comprisingdiscontinuing flow of overhead gases from the oxidation reactor into theseparation device, and treating the device with the solvent compositionof the present invention in the substantial absence of molecular oxygen.Discontinuing flow of reactor off gas to the separation device normallyoccurs during process shutdowns; however, in processes in which two ormore separation systems are used, it also may be suitable to isolate oneor more such system for treatment while one or more others continueoperating. An atmosphere substantially free of oxygen is convenientlyestablished in a distillation column or other separation device bypurging or maintaining a flow of an inert gas such as nitrogen throughthe device. Oxygen levels during treatment should be maintained lowenough to avoid formation of potentially flammable mixtures.

For use to remove iron oxide deposits from the internal titaniumcomponent surfaces of a distillation column or similar separation device(including surfaces of internal titanium packing materials), the solventcomposition may be suitably introduced at the top of a distillationcolumn and allowed to flow downwards and distribute throughout thecolumn thereby contacting the column and the packing materials. Sprayingor dripping the solvent from the top of the column onto the packing witha suitable spray or other liquid distribution means, including thosenormally used for introducing reflux liquid at the top of the column,are effective for introducing the solvent or its components into thecolumn so that the liquid can flow downwardly through the column,typically by force of gravity, into contact with other packing surfaces.Preferably, before introduction to the column, the column and thesolvent solution are purged with an inert gas such as nitrogen in orderto remove reactive oxygen. After flowing downwards, solvent compositionis collected at the bottom of the distillation column and can bedisposed of or recycled to the top of the column, with addition ofmakeup fresh or recycle solvent as may be appropriate, by feeding andintroducing it again to the top of the distillation column. The solventsolution may be recycled continuously through the column or otherseparation device or it can be recycled numerous times in a batch mode.Rinsing the titanium components with water after circulation of thesolvent composition through the column or device is completed isbeneficial for removal of solvent and residues thereof.

For use in treating a distillation column or other separation device,the solvent composition may be heated. Suitable temperatures atatmospheric pressure include between 10° C. to 135° C., preferably fromabout 30 to about 125° C. more preferably between 55° C. to 100° C., andeven more preferably between 70° C. to 90° C. Contacting the titaniumcomponent internal components of the separation device is mostpreferably conducted at atmospheric pressure.

In a more specific embodiment of the invention, the solvent compositionis contained by a reservoir, such as a holding tank, and is pumped fromthe reservoir to the top of a distillation column and allowed to flowdownwards and distribute throughout. When the solvent compositionreaches the bottom of the column it, including soluble iron compoundsdissolved therein, is returned to the reservoir where the composition iscollected and recycled, with makeup solvent or components, by pumping itback to the top of the distillation column.

In another specific embodiment, the solvent composition is created bysequentially introducing an organic acid, which may be aqueous andhydrohalide acid, preferably in aqueous form, to a distillation column.In this embodiment, the organic acid, preferably after heating, isintroduced to the top of a distillation column and allowed to flowdownwardly and distribute throughout the column thereby contacting thecolumn and packing materials. The organic acid is introduced with awater content preferably less than 10 wt. %, more preferably less than 5wt. %, and even more preferably less than 1 wt. %. Before itsintroduction to the top of the distillation column, both the organicacid and the column are purged with an inert gas such as nitrogen inorder to remove reactive oxygen. When the organic acid reaches thebottom of the distillation column, it can be recycled for recirculationthrough the column by introducing it to the top of the distillationcolumn. While the organic acid is passed through the column, hydrohalideacid is introduced to the distillation column or to a reservoir used tocontain the organic acid, thereby forming the solvent composition inaccordance with the invention. This solvent composition is thencirculated, with recycle if desired, through the column. The hydrohalideacid is introduced as an aqueous solution preferably comprising water inan amount greater than 30 wt. %, more preferably greater than 40 wt. %,and even more preferably about 45 to 55 wt. %. The organic acidpreferably is heated at a first temperature before introduction into thecolumn, such as by contact with a heat exchange surface. Heating isdiscontinued and the heat exchange surface is allowed to coolsufficiently before addition of hydrohalide acid so that the acid doesnot damage the surface. After the hydrohalide acid is added, theresulting solvent composition cools to a second temperature. Typically,the second temperature is at least about 10° C. lower than the firsttemperature. The first temperature is preferably between 30° C. to 135°C., more preferably between 55° C. to 100° C., and even more preferablybetween 80° C. to 100° C. The second temperature is preferably between30° C. to 110° C., more preferably between 55° C. to 100° C., and evenmore preferably between 70° C. to 90° C. The amounts of organic acid,hydrohalide acid and their respective water contents are chosen toachieve a desired resultant solvent composition after their sequentialintroduction to the column. The organic acid present in the resultantsolvent composition is preferably greater than 50 wt. %, more preferablygreater than 75 wt. %, and even more preferably from 70–90 wt. %. Thehydrohalide acid present in the resultant solvent composition ispreferably between 0.5 to 20 wt. %, preferably from about 1 to 10 wt. %,and more preferably from 4 to 8 wt. %. The water present in theresultant solvent composition is preferably between 0.5 to 25 wt. %,more preferably from about 3 to 15 wt. %, and more preferably from 4 to8 wt. % of the total weight of the solvent composition.

The invention is further described with reference to the drawing. Shownin the drawing FIGURE is a distillation column 20, having a top at 20 aand a bottom at 20 b, with titanium component internal packing materials21. In accordance with the invention, an organic acid, preferably aceticacid, is contained in a reservoir 22. Preferably the organic acid has nogreater than 1 wt. % water. Both the column 20 and the reservoir 22containing the organic acid are purged with an inert gas, preferablynitrogen. The organic acid is pumped to the top of the distillationcolumn 20 a using a pump (not shown). The organic acid is sprayed ordripped onto internals of the column at the top thereof and allowed todescend by force of gravity downwardly through column 20 and passesover, around and between packing materials 21. The organic acid iscollected from the bottom 20 b of the distillation column and returnedto the reservoir 22 where it is recycled by pumping it back into thecolumn 20. The organic acid is heated to a first temperature, mostpreferably between 80° C. to 100° C., by a heating apparatus 23, whichis preferably a heat exchanger. Although heating apparatus 23 is shownin the drawing as disposed in the line feeding the top of thedistillation column, any heating apparatus suitable for heating theorganic acid can be used. Once the first temperature is achieved, thereis preferably no further heating and, more preferably the heat exchangesurface is allowed to cool sufficiently before addition of hydrohalideacid so that the acid is not heated in contact with the heat exchangesurfaces to a temperature high enough to damage the surfaces.Hydrohalide acid, preferably hydrobromic acid, is then introduced fromsource 24 and pumped into reservoir 22 to mix with the organic acidthereby forming the solvent composition of the invention. Preferably,the total amount of hydrohalide acid is added slowly over an extendedperiod of time (e.g. 1–4 hours or more) in order to promote equilibriummixing with the organic acid. The solvent composition is then circulatedto and through the distillation column 20. Typically the solventcomposition is circulated for a period of from 1 to 48 hours. For bestresults, all of the surfaces of the components which come in contactwith the solvent composition should comprise titanium or other materialwhich is substantially insoluble in the solvent composition.

The organic acid originally contained in 22 is preferably a mixturecomprising water and primarily organic acid wherein the water content ofthe mixture is preferably less than 10 wt. %, more preferably less than5 wt. %, and even more preferably less than 1 wt. %. The hydrohalideacid originally contained in source 24 is preferably an aqueous solutionpreferably comprising water in an amount greater than 30 wt. %, morepreferably greater than 40 wt. %, and even more preferably about 45 toabout 55 wt. %. Most preferably the hydrohalide acid is aqueous 48 wt. %hydrobromic acid.

In a specific embodiment of the invention, titanium components such aspacking materials are treated with the solvent composition to controliron oxide deposits on surfaces of the components so that deposits donot accumulate to levels that may adversely affect the titaniumcomponents. Thus, treatment according to the invention can be includedin maintenance procedures used during shutdowns or other interruptionsof processes in which equipment with titanium components is used.Thickness of solid iron oxide deposits on surfaces of titaniumcomponents can easily be maintained at less than 300 microns, andpreferably less than 100 microns, more preferably less than 50 microns,and even at less than 25 microns by treatment according to theinvention, including repeated treatments as may be appropriate toequipment used in a given process under its particular conditions.

Accordingly, for example, the method and solvent composition of thisinvention may be integrated with a process for the manufacture of anaromatic carboxylic acid. In general, the method and solvent compositionof the present invention may be integrated with a process for themanufacture of an aromatic carboxylic acid comprising the steps ofoxidizing an aromatic compound to an aromatic carboxylic acid in aliquid phase reaction mixture comprising the aromatic compound, water, alow molecular weight monocarboxylic acid solvent, an oxidation catalystand a source of molecular oxygen, under reaction conditions, to producean aromatic acid product and a gas phase comprising water vapor,unreacted molecular oxygen and gaseous low molecular weightmonocarboxylic acid solvent; removing all or part of the gas phase fromthe reaction zone to a separation device having internal titaniumcomponents; and separating the gas phase in the separation device into aliquid phase comprising monocarboxylic acid solvent and a second gasphase comprising water vapor. The oxidation is carried out in a reactionzone, such as provided by a closed, pressure-rated stirred tank reactoror vessel, with sufficient volume to accommodate the liquid reactionmixture and the gaseous phase, with the latter most commonly present asan overhead gas phase above the liquid level in the reaction zone.

In a specific embodiment of such a process, a feed material comprisingpara-xylene is oxidized to product comprising terephthalic acid using asolvent comprising acetic acid and air as a source of molecular oxygen.Catalysts for such processes typically comprise a heavy metal component,and most preferably cobalt, manganese and a source of bromine.

The gas phase is removed from the reaction zone to the separationdevice, which preferably is, or includes, a distillation apparatus withtitanium packings for separation of the gas phase into liquid andgaseous phases. Monocarboxylic acid in the liquid phase is convenientlyrecycled to the reaction zone. The remaining gas phase can be condensedto recover water for other uses, used for heat exchange or directed to asuitable energy recovery device, such as an expander.

Examples of processes for the manufacture of an aromatic carboxylic acidin which the present invention can be utilized are disclosed in U.S.Pat. No. 5,925,786; U.S. Pat. No. 5,723,656; U.S. Pat. No. 5,510,521;U.S. Pat. Nos. 5,463,113; 5,621,007; and British Patent 1373230; all ofwhich are incorporated by reference.

The separation device is treated to remove solid iron oxide depositswhich may be present on a surface of its internal titanium components bydiscontinuing removal of the gas phase from the reaction zone to theseparation device and separation of the gas phase therein, and passingthrough the device in contact with the titanium component surfaces andin the substantial absence of molecular oxygen the aqueous solventcomposition according to the invention. Discontinuing the removal andseparation steps can be effected by any suitable means, such as inconjunction with process shutdowns or interruptions. As described above,in processes conducted with multiple separation devices it may also besuitable to bypass or isolate one or more of such device, which, afterevacuation of process materials, can be treated while one or more otherdevices continues operating. Evacuation of process materials from theseparation device and rinsing the device, for example with caustic,water, or both in sequence, preferably is conducted before treating thedevice with solvent composition for removal of iron oxide deposits thatmay be present. Purging the device with inert gas before treating withthe solvent composition also is beneficial toward maintaining anatmosphere substantially free of molecular oxygen.

As will be appreciated, passage of the solvent composition or itscomponents through the separation device is carried out in thesubstantial absence of molecular oxygen to avoid creation of potentiallyflammable mixtures within the device. An atmosphere substantially freeof molecular oxygen refers to presence of molecular oxygen at low enoughlevels, taking into account organic species present during treatmentwith the solvent composition, iron oxide levels that may be present, andconditions such as temperature and pressure, to avoid formation of aflammable vapor mixture in the device. Persons skilled in the art candetermine such levels by reference to standard sources concerningflammable gas mixtures. Most conveniently, a substantial absence ofmolecular oxygen is maintained in the separation device by purging thedevice with an inert gas, such as nitrogen, before introducing thesolvent composition or its components, by maintaining a flow of inertgas through the device during treatment, or both.

As described above, solvent composition can be formulated before beingintroduced to the separation device or its components can be addedseparately. In a particular embodiment, passage of the solventcomposition through the separation device is accomplished by passing theorganic acid component, optionally with water, through the device in thesubstantial absence of molecular oxygen, adding hydrohalide acid andwater to the organic acid in the substantial absence of molecular oxygento obtain the solvent composition; and passing the solvent compositionthrough the device in contact with the titanium component in thesubstantial absence of molecular oxygen. Best results in such anembodiment are attained when the organic acid is heated to a firsttemperature, preferably of about 40 to about 125° C. and passed throughthe device substantially at the first temperature, after which theaqueous hydrohalide acid at a lower temperature is added to the organicacid, such that the acids combine to form the solvent composition at alower temperature and are passed through the device into contact withthe titanium components in the substantial absence of molecular oxygen.Preferred solvent compositions for such use contain about 4 to about 8wt. % hydrohalide acid, about 5 to about 15 wt. % water and about 77 toabout 91 wt. % organic acid, with hydrobromic and acetic acids beingmost preferred as the hydrohalide and organic acids, respectively.

In processes in which the low molecular weight monocarboxylic acidreaction solvent also is a material suitable as the organic acidcomponent of the solvent composition according to the invention, use ofthe same acid as both the reaction solvent and solvent compositioncomponent can provide further advantages. For example, in themanufacture of terephthalic acid by oxidation of a para-xylene feed inacetic acid as the reaction solvent, use of acetic acid as the organicacid of the solvent composition for treating the separation device canfacilitate solvent handling and purification. Removal of solvent withdissolved iron compounds after passage through the separation device,and recovery of organic acid therefrom, such as by distillation, can beemployed to allow recycle of the organic acid for use as reactionsolvent. Conversely, liquid monocarboxylic acid solvent recovered as aresult of separation of the gas phase removed from the reaction zone canbe recycled for use as the organic acid component of the solventcomposition for treating the titanium internal components of theseparation device.

The following examples illustrate, but do not limit, the invention.

EXAMPLE 1

Water, acetic acid, and HBr were charged into a round bottom four-neckedflask equipped with a condenser and stirrer in the following amounts: 13g water, 85 g acetic acid, and 2 g aqueous HBr (48 wt. % HBr). Thisyielded a solvent composition of 14.04 wt. % water, 85 wt. % aceticacid, and 0.96 wt. % HBr. The solution was purged gas by bubblingnitrogen gas into the solution in the flask. While stirring, 0.30 g ofiron (III) oxide was charged into the flask. The contents of the flaskwere heated and maintained at 80° C. under a constant nitrogen gaspurge. After 24 hours under these conditions, undissolved iron (III)oxide was observed. In order to recover undissolved iron (III) oxide,the flask contents were filtered hot (85° C.) at reduced pressure (15in. Hg, 381 mm Hg) using a vacuum flask, a filtering funnel, and 5.5 cmWHATMAN filter paper #1 (Cat. No. 1001.055) from Whatman InternationalLtd. of the United Kingdom. The amount of recovered iron (III) oxide was0.025 g, corresponding to 8.63 wt. % of the 0.30 g of iron (III) oxidethat was initially charged.

EXAMPLES 2–6

The procedures of Example 1 were substantially repeated but with thevariations reported in Table 1.

Results of Examples 1–6 are summarized in Table 1 below and show thatvarious solvent compositions comprising water, acetic acid, andhydrobromic acid were effective for dissolving iron (III) oxide.

TABLE 1 100%- wt % Undissolved Undis- HBr H2O Fe₂O₃ Temp Time Fe₂O₃solved Ex (wt %) (wt %) (g) (° C.) (hr) (g/wt %) Fe₂O₃ 1 0.96 14.04 0.3080 24  0.025/8.63 91.37 2 0.96 14.04 0.30 108 52 0.0167/5.57 94.43 32.88 12.12 0.30 108 0.2 Nil ~100  4* 2.88 12.12 0.60 102 10.50.0167/2.73 97.27 5 2.88 12.12 0.90 108 10.25 0.0415/4.61 95.39 6 2.8812.12 1.5 108 15.5  0.101/6.73 93.27 *The contents of the flask were notstirred in this example.

EXAMPLE 7

Water, acetic acid, and HBr were charged to a round bottom, four neckedflask equipped with a condenser and stirrer in the following amounts:18.0 g water, 170.0 g acetic acid, and 12.0 g aqueous HBr (48 wt. %HBr). This yielded a solvent composition of 12.12 wt. % water, 85 wt. %acetic acid, and 2.88 wt. % HBr. The solution was purged with nitrogenas in the prior examples. While stirring, 3.0 g of iron (III) oxide wascharged into the flask and heated and maintained at 30° C. About a 3gram sample of the solvent composition was taken. The contents of theflask were then stirred and maintained at temperature of 30° C. Samplesof the solvent composition were taken at two-hour intervals afterstirring was stopped and undissolved iron (III) oxide was allowed tosettle. After each sampling, stirring of the flask contents was resumed.Samples were taken over an eight-hour period. All samples were analyzedfor Fe content by Inductively-Coupled Plasma Spectrometry (ICP).Results, designated 7a, are summarized in Table 2. Undissolved iron(III) oxide remaining after the sampling period was filtered hot (85°C.) at reduced pressure (15 in. Hg, 381 mm Hg) using a vacuum flask, afiltering funnel, and 5.5 cm WHATMAN filter paper #1. The amount ofundissolved iron (III) oxide was recorded in grams and divided by 3.0 g(initial amount of iron charged) to calculate the wt. % undissolved iron(III) oxide.

This procedure was repeated except the solution was maintained at 60° C.while stirring. Results, designated 7b, are reported in Table 2.

The procedure was repeated again but with a 1.5 g iron (III) oxidesample and the temperature maintained at 108° C. for 15.5 hours whilestirring. Results, designated 7c, are reported in Table 2 and alsodiscussed after the table.

The results for examples 7a–7c illustrate effects of time andtemperature on effectiveness of the solvent composition for dissolvingiron (III) oxide.

TABLE 2 Fe by Undissolved Dissolved Fe₂O₃ Temp. Time ICP Fe₂O₃ at 8 hrs.at 8 hrs. Ex (° C.) (hrs) (ppm) (g) (wt. %) (wt. %) 7a 30 0 359 1.43847.93 52.07 2 3430 4 3810 6 4110 8 4250 7b 60 0 2330 1.0869 36.23 63.772 4830 4 4220 6 5770 8 5680 7c 108 0 5090 0.293 19.53 80.47 2 6715 47930 6 7910 8 8490

After 15.5 hours under the conditions of Example 7c, undissolved ironoxide was observed and was recovered by filtering the contents of theflask hot (85° C.) at reduced pressure (15 in. Hg, 381 mm Hg) using avacuum flask, a filtering funnel, and 5.5 cm WHATMAN filter paper #1.The amount of recovered iron (III) oxide was 0.101 g, which corresponds6.73 wt. % of the 1.5 g of iron (III) oxide that was initially charged.

EXAMPLE 8

A 100 g sheet of corrugated titanium packing material designated GEMPACK2A, Titanium grade 1, having a thickness of 0.10 mm was obtained fromKock-Glitsch Inc. The sheet was cut into one inch (2.54 cm) squaresamples. Acetic acid (1575 g) and aqueous, 48 wt. % HBr (225 g) werecharged to a two liter container equipped with a condenser and stirrer.This yielded a solvent composition of 6.5 wt. % water, 87.5 wt. % aceticacid, and 6.0 wt. % HBr. The solution was purged with nitrogen as in theprevious examples. While stirring, all of the square samples werecharged into the container and the contents were heated and maintainedat 80° C. while maintained under a constant nitrogen purge. A sample(about 5 g) of the solvent composition was taken. The contents of thecontainer were then stirred and maintained at temperature of 80° C.under constant nitrogen purge. Samples (about 5 g) of the solventcomposition were taken after 1 hour, 2 hours, 4 hours, 8 hours, 24hours, and 48 hours. Before each sampling, stirring was stopped and thecontents of the container were allowed to come to rest. After eachsampling, stirring of the flask contents was resumed. All samples wereanalyzed to determine the amount of dissolved titanium by ICP. After 48hours only 9.9 ppmw of titanium was dissolved in the solventcomposition, which corresponds to 0.018 wt. % of the original 100 gsample of titanium packing material. Results of sampling at differenttimes are summarized in Table 3 and show that the solvent compositioncomprising water, acetic acid, and hydrobromic acid did notsubstantially dissolve titanium metal.

TABLE 3 Amount of Time Dissolved Titanium (hrs) (ppmw) (wt %) 0 0.20.00002 1 1.9 0.00019 2 3.5 0.00035 4 4.4 0.00044 8 6.5 0.00065 24 7.90.00079 48 9.9 0.00099

EXAMPLES 9–13 AND COMPARATIVE EXAMPLE 14

Titanium samples with iron oxide deposits were used in Examples 9–13.The titanium samples had a layer of iron oxide deposits in the range of15 μm to 25 μm as determined by scanning electron microscopy (SEM).About 90 wt. % of the iron oxide deposits was iron (III) oxide asdetermined by x-ray diffraction (XRD) and energy dispersive spectrometry(EDS). The titanium samples were obtained from packing materials whichhad been used in a distillation column of a commercial-scale unit formanufacture of terephthalic acid by liquid phase oxidation of apara-xylene feed. The distillation column had been operated on top of aliquid phase oxidation reactor and used to separate acetic acid fromwater in a reactor off-gas removed from the reactor into the column. Thepacking materials were corrugated titanium packing materials, Titaniumgrade 1, 0.10 mm thickness, similar to the unused sample in Example 8.Examples 9–10 show that solvent compositions comprising water, aceticacid, and hydrobromic acid were effective for removing iron oxidedeposits from the used titanium packing material. Examples 11a–11c showhow the amount of iron in solution affected the ability of the solutionto remove an iron oxide coating from the surface of the titaniumcomponent. The solution from 11a with little or no iron present was veryeffective. However, the solution from 11c that had significantly moreiron and was near its maximum iron uptake quantity did not effectivelyremove the iron oxide coating. Example 12 shows the use of hydrochloricacid in place of hydrobromic acid. Example 13 shows the use of propionicacid in place of acetic acid. Comparative Example 14 shows that a commonsolvent for removing iron oxide, oxalic acid dihydrate, also dissolvedsignificant titanium.

EXAMPLE 9

Water (218.75 g), acetic acid (2125 g), and aqueous 48 wt. % HBr (156.25g) were mixed to obtain a solvent composition of 12 wt. % water, 85 wt.% acetic acid, and 3 wt. % HBr. A 1200 ml column equipped with acondenser and band heaters was erected laterally and was filled with thesolvent composition. A nitrogen atmosphere was maintained in theheadspace over the liquid level in the column. The solvent compositionwas then heated and maintained at 60° C. in the column. A sample cutfrom the titanium packing material with iron oxide deposits was weighedto record an initial weight and was supported horizontally in a funnel.The wash solution was allowed to flow from the column and over thesample at an average flow rate of 210 ml/hr for 20 hours. Thetemperature at which the solvent composition contacted the sample was45° C. and was measured by placing a thermocouple in line of the solventcomposition liquid flow at the point of contact with the sample. After20 hours, the sample was removed, dried and weighed. The weight losscalculated from the initial and final weights in this example, referredto as 9a, was 8.05 wt. %.

The procedure of Example 9a was repeated but at a solvent temperature inthe column of 30° C., contact temperature of 25° C. and for 22.5 hours.The calculated weight loss for this example, referred to as 9b, was 3.33wt. %.

After sampling in 9b, heating was continued, with average flow rate of215 ml/hr., and a sample was taken after heating for a total of 34hours. The calculated weight loss for the sample, referred to as 9c, was4.27 wt. %.

After sampling in 9c, heating was again continued, with an average flowrate of 212 ml/hr. A sample, designated 9d, was taken after the totalheating time was 41 hours. The calculated weight loss for this example,referred to as 9d, was 4.81 wt. %.

Results of these examples are reported in Table 4. After contact withthe solvent compositions, visual observation of the samples from 9a –drevealed decreased levels of iron oxide on the sample surfaces with noor negligible damage to the titanium surfaces, indicating weight losseswere attributable essentially to solid iron oxide dissolved from thesample surfaces.

TABLE 4 Temp. (° C.) Flow Rate Time Weight Loss Ex. Column Contact(ml/hr) (hrs) (wt %) 9a 60 45 210 20 8.05 9b 30 25 210 22.5 3.33 9c 3025 215 34 4.27 9d 30 25 212 41 4.81

EXAMPLE 10

Water (104.56 g), acetic acid (1015.75 g), and aqueous 48 wt. % HBr(74.68 g) were mixed to obtain a solvent composition of 12 wt. % water,85 wt. % acetic acid, and 3 wt. % HBr. A 1200 ml column equipped with acondenser and band heaters was erected laterally and was filled with thesolvent composition. The solvent was heated to and maintained at 80° C.in the column. A sample cut from the iron oxide-coated titanium packingmaterial was weighed to record an initial weight and was supportedhorizontally in a funnel. The wash solution was allowed to flow from thecolumn and over the sample at an average flow rate of 208 ml/hr. Thetemperature at which the solvent composition contacted the sample was59° C. and was measured as in Example 9. At intervals of 4 hours, 11.5hours, 18.25 hours, and 21.25 hours the flow was stopped and the samplewas removed from the funnel and weighed to determined weight loss. Afterweighing, the sample was returned to the funnel and the process wasresumed. Weight loss at each time interval is shown in Table 5. After21.25 hours, no visible iron oxide deposits were observed on thetitanium sample.

TABLE 5 Time (hrs) Weight Loss (wt %) 4 5.30 11.5 5.84 18.25 18.04 21.2519.19

The high percentage weight losses at 18.25 and 21.25 hours wereattributable to heavy iron oxide coatings on the sample material.

EXAMPLE 11

Acetic acid (87.5 g) and 48 wt. % HBr (12.5 g) were charged to a roundbottom four necked flask equipped with a condenser and stirrer. Thisyielded a solvent composition of 6.5 wt. % water, 87.5 wt. % aceticacid, and 6.0 wt. % HBr. The solution was purged with nitrogen gas as inprevious examples. While stirring, an iron oxide-coated sample of thetitanium packing material weighing 0.2473 g was charged into the flask.The contents of the flask were heated, stirred, and maintained at 80° C.under a constant nitrogen gas purge. After 48 hours, the sample wasshiny and visually free of iron oxide deposits. The solution comprised0.7 ppm of dissolved titanium and 143 ppm of dissolved iron asdetermined by ICP.

In another trial, water (2.42 g), acetic acid (87.5 g), aqueous, 48 wt.% HBr (7.8465 g), and FeBr₃ (2.754 g) were charged into a round bottomfour-neck flask equipped with a condenser and stirrer. This yielded asolvent composition of 6.5 wt. % water, 87 wt. % acetic acid, 3.7 wt. %HBr, and 0.52 wt. % iron. The solution was purged with nitrogen gas asin previous examples. While stirring, a sample of the iron oxide coatedpacking (0.3312 g) was charged to the flask. The contents of the flaskwere heated, stirred, and maintained at 80° C. under a constant nitrogengas purge. After 48 hours, the sample was shiny and visually free ofiron oxides. The solution comprised 0.8 ppm of dissolved titanium and5300 ppm of dissolved iron as determined by ICP.

In another trial, water (5.586 g), acetic acid (87.5 g), aqueous 48 wt.% HBr (1.758 g), and FeBr₃ (6.356 g) were charged into a round bottomfour necked flask equipped with a condenser and stirrer. This yielded asolvent composition of 6.4 wt. % water, 86.5 wt. % acetic acid, 0.83 wt.% HBr, and 1.2 wt. % iron. The solution was purged with nitrogen gas asin the previous trials. While stirring, a sample of the iron oxidecoated titanium packing (0.3125 g) was charged to the flask. Thecontents of the flask were heated, stirred, and maintained at 80° C.under a constant nitrogen gas purge. After 48 hours, iron oxide depositswere still visible on the sample but the solution contained 0.6 ppm ofdissolved titanium and 12160 ppm of dissolved iron as determined by ICP.

EXAMPLE 12

Water, acetic acid, and aqueous HCl were charged to a 250 ml roundbottom four neck flask equipped with a condenser and stirrer in thefollowing amounts: 6.892 g water, 85 g acetic acid, and 8.108 g aqueousHCl (37 wt. % HCl). This yielded a solvent composition with 12.0 wt. %water, 85 wt. % acetic acid and 3.0 wt. % HCl. The stirred solvent waspurged with nitrogen and heated to 60° C. A sample of the titaniumpacking with iron oxide coating was added to the hot solution. After sixhours under these conditions, the titanium sample was shiny and visuallyappeared to have no iron oxides. The sample was weighed to determine arecorded weight loss of 0.0129 g, which corresponds to 8.6 wt. % basedon initial sample weight.

EXAMPLE 13

Water, propionic acid, and HBr were charged to a 250 ml round bottomfour neck flask equipped with a condenser and stirrer in the followingamounts: 12.0 g water, 85.0 g propionic acid and 6.0 g aqueous HBr (48wt. % HBr). This yielded a solvent composition containing 14.68 wt. %water, 82.52 wt. % propionic acid, and 2.80 wt. % HBr. The stirredsolution was purged with nitrogen and heated to 60° C. A sample of thetitanium packing coated with iron oxide was added to the hot solution.The sample was removed and weighed at 8, 32, 40, and 56 hours todetermine the rate of iron oxide removal. After 56 hours, 0.0071 g or4.21 wt. % (75.53 wt. % of 0.0094 g total coating weight) was removedfrom the titanium packing. The solution was heated to 80° C. and thetitanium sample was contacted at this condition for another 2.5 hoursand then weighed. The titanium packing was free of iron oxide visibly.At the end of the additional 2.5 hours, the sample was free of visibleiron oxide and weight loss was 0.0094 g, which corresponds to 5.58 wt. %of the initial sample. Results of the sampling in this example arereported below.

Fe₂O₃ Weight Loss Time (hours) Temp (° C.) (grams) Wt. % 8 60 0.00412.43 32 60 0.0058 3.44 40 60 0.0062 3.68 56 60 0.0071 4.21 Temperaturewas increased to 80° C. 1.5 80 0.0088 5.20 2.5 80 0.0094 5.58

COMPARATIVE EXAMPLE 14

Water (17,660 g) and 99% purity oxalic acid dihydrate (883 g) werecharged to a 19 liter glass tank with nine port holes and equipped witha condenser and stirrer to provide a solvent composition containing96.60 wt. % water and 3.40 wt. % oxalic acid. Three new titaniumcorrugated packing bundles (GEMPACK 2A, Titanium grade 1, 0.10 mmthickness commercially obtained from Kock-Glitsch Inc.) wereindividually weighed and recorded. Each titanium packing bundle was madeup of six pre-cut sheets that weighed between 5.0–6.3 g. The bundleswere labeled Bundle #1, Bundle #2 and Bundle #3. The aqueous oxalic acidsolution was stirred and heated to 80° C. The titanium bundles weresuspended in the hot solution and periodically weighed. The titaniumweight loss of Bundle #1 was 3.92 g (11.13 wt. %) after 3 hours and 5minutes. The titanium weight loss of Bundle #2 was 8.704 g (26.12 wt. %titanium) after 5 hours and 40 minutes. The titanium weight loss ofBundle #3 was slightly below 26 wt. % after 5 hours and 40 minutes.After an additional 1.5 hours in the solvent, a weight loss of slightlyabove 26 wt. % for Bundle #3 was recorded. A new batch of solvent (17660g of water and 883 g of oxalic acid dihydrate) was made and heated to80° C. Bundle #3 was suspended in the hot solution for an additional 4hours and 53 minutes. Bundle #3 weight loss after that period was 17.769g (50.47 wt. %).

Iron oxide coated titanium packing samples were also tested with theoxalic solution but removal of iron oxides was accompanied bysubstantial dissolving of titanium metal.

As many different embodiments of this invention may be made withoutdeparting from the spirit and scope thereof, it is to be understood thatthe invention is not limited to the specific embodiments thereofdescribed herein.

1. A method for treating a gas-liquid separation device comprising aninternal titanium component to control accumulation of deposits of solidiron oxide on a surface of the titanium component comprising the stepsof (a) introducing to the separation device an organic acid comprisingan alkyl monocarboxylic acid having 2 to 6 carbon atoms, benzoic acid ormixture thereof at a first temperature in the range of about 30 to about125° C. and passing the organic acid through the separation device inthe absence of molecular oxygen levels forming a flammable mixture; (b)adding to the organic acid in the absence of molecular oxygen levelsforming a flammable mixture an aqueous hydrohalide acid at a temperaturewhich is lower than the first temperature to obtain an aqueous solventcomposition at a second temperature which is lower than the firsttemperature; and (c) contacting the titanium component with the solventcomposition in the separation device in the absence of molecular oxygenlevels forming a flammable mixture.
 2. The method of claim 1 wherein theorganic acid is an aqueous alkyl carboxylic acid of 2 to 6 carbon atoms.3. The method of claim 1 wherein the organic acid is acetic acid.
 4. Themethod of claim 1 wherein the organic acid is benzoic acid.
 5. Themethod of claim 1 wherein the hydrohalide acid is hydrobromic acid. 6.The method of claim 1 wherein the aqueous hydrohalide acid contains atleast about 30 wt. % hydrobromic acid.
 7. The method of claim 1comprising an additional step comprising maintaining a flow of an inertgas through the separation device.
 8. The method of claim 1 wherein thegas-liquid separation device comprises a distillation column and aninternal titanium component comprises titanium packings or trays.
 9. Themethod of claim 1 wherein the gas-liquid separation device comprises areflux condenser and an internal titanium component comprises titaniumpackings or trays.
 10. The method of claim 6 wherein the organic acidand the hydrohalide acid are introduced in amounts such that the solventcomposition contains between 0.5 to 20 wt. % hydrohalide acid andbetween 0.5 to 25 wt. % water.
 11. The method of claim 8 wherein theorganic acid comprises acetic acid.
 12. The method of claim 9 whereinthe organic acid comprises acetic acid.
 13. The method of claim 9wherein the organic acid comprises hydrobromic acid.
 14. The method ofclaim 10 wherein the second temperature is between 55° C. to 100° C. 15.The method of claim 11 wherein the hydrohalide acid compriseshydrobromic acid.